1. Field of the Invention
The present invention relates to an electronic camera and an automatic focusing method.
2. Description of the Related Art
Electronic image pickup apparatus, or so-called digital cameras, formed by using a CCD (charge coupled device) image sensor as photoelectric conversion elements are being popularly marketed.
Such digital cameras are generally equipped with an auto focus control feature of automatically focusing the camera lens. While there are a number of auto focus control systems available to digital cameras, the system of generating an estimated in focus value by extracting the contrast, the high frequency components and the like of the detection range of the image to be picked up and adjusting the lens position so as to maximize the estimated in focus value is currently in the main stream.
FIG. 1 shows a graph of estimated in focus value relative to a lens position. Referring to FIG. 1, the estimated in focus value (Y-axis) is maximized relative to the lens position (X-axis) at the in focus position and decreases as a function of the distance from the in focus position toward the far side (forwardly out of focus) and the near side (backwardly out of focus) to show a hilly curve. Therefore, for auto focus control, it is only necessary to control the lens position of the focusing lens in such a way that the estimated in focus value is found at the top of the hilly curve.
When moving the focusing lens so as to bring the estimated in focus value to the top of the hilly curve, it is necessary to detect the gradient of the hilly curve at the point where the estimated in focus value is found and determine the current lens position relative to the in focus position. That is, it is necessary to determine if the current lens position is at the far side or at the near side relative to the in focus position.
FIG. 1 also shows a proposed method for determining the current lens position relative to the in focus position. Referring to FIG. 1, according to the proposed method, the focusing lens is subjected to micro-oscillations to such an extent that they do not affect the image to be picked up in order to computationally determine the positiveness or negativeness of the fluctuating component of the estimated in focus value (differential component dy/dx) (see, inter alia, Patent Document 1: Japanese Patent Application Laid-Open Publication No. 10-239579). The operation of subjecting the focusing lens to micro-oscillations for auto focus control is referred to as wobbling.
A specific example of wobbling operation will be described by referring to FIG. 2. In the graph of FIG. 2, the horizontal axis represents time (field unit) and the vertical axis represents the focusing lens position.
In the wobbling operation, the focusing lens is moved toward the near side in the first time period (T1) by a predetermined distance. Subsequently, the focusing lens is held standing still for a predetermined period in the second time period (T2). Then, the focusing lens is moved toward the far side in the third time period (T3) by a predetermined distance. Finally, the focusing lens is held standing still again for a predetermined period in the fourth time period (T4). Then, the sequence of move and stop sessions of the first through fourth time periods is repeated for the wobbling operation. The estimated in focus value is detected both in the second time period and the fourth time period and the difference of the two values is computed in order to detect the fluctuating component (differential component dy/dx) in the wobbling operation.